Factors influencing bone formation and resorption are examined in organ culture and in vitro. In addition to the established hormone, parathyroid hormone, active metabolites of vitamin D, and calcitonin, new agents including prostaglandins and a newly-discovered bone resorbing factor from leukocytes, osteoclast activating factor, will be examined. In addition to these stimulators of bone resorption, which are also often inhibitors of bone formation, other agents such as calcitonin, glucocorticoids, and drugs such as diphosphonates are being studied. In vivo experiments are planned in animals to assess the importance of these various factors in the development and treatment of bone lesions and cancer and inflammation as well as metabolic bone disease.